1. Field of the Invention
This invention relates to ice-breaking ships in general, or, specifically, to ships having hulls that are adapted to break ice for the purpose of forming substantially ice-free channels through ice fields.
2. Description of the Prior Art
The prior art presents many concepts for the design of ice breaking hulls. For example, icebreaker hulls have been disclosed in the following U.S. Pat. Nos.: 17,209, issued May, 1857 to Estlack; 151,774, issued June, 1874 to Grant; 593,664, issued November, 1897 to Inman; 812,656, issued February, 1906 to Julhe; 857,766, issued June, 1907 to Stangebye; 3,636,904, issued January, 1972 to Blanchet; 3,850,125, issued November, 1974 to Anders; and 3,934,529, issued January, 1976 to Gallagher.
Over the years, many icebreaker designs have appeared. These designs have included features to permit plowing, pounding, sawing, or combinations thereof. Recently, patents have been granted for icebreaker hulls having explosive-type features and those that produce bending of the ice sheet by inducing and/or regulating the pitching motion of the breaker. The eight U.S. patents previously cited, are representative of such designs and maneuvers. Of these, only three (U.S. Pat. Nos. 17,204, 151,774, and 857,766) disclose removing large portions of the broken ice from the water. U.S. Pat. Nos. 17,204 and 151,774 involve adjustable, removable attachments for merchant ship hulls; 857,766 discloses a structure that is integral with the hull of a ship. However, defining the limitations of these disclosed invention is difficult since no model or full scale tests appear to have been conducted.
The most successful design, and that currently used throughout the world, is typified by the newest U.S. Coast Guard icebreakers of the "Polar Star" class. The following principal features are characteristic of this type of ship:
a. A length-to-beam ratio of about 4:1;
b. A flared hull from bow-to-stern;
c. The maximum beam located up to about 1/3 of the total length of the ship, measured from the stem, aft; and
d. A highly raked stem particularly below the water line.
Contemporary ice breaking is performed by icebreakers in two principal modes:
a. The continuous mode in which the ship is driven forward through the ice at varying speeds (restrained only by the ice resistance), but during which forward movement is never totally impeded. The severely-raked stem and flared forebody rise partially on the ice sheet, crush the ice under the forefoot, and move through the broken ice that remains. This process is repeated continuously.
b. The ramming mode results when the icebreaker encounters ice of such thickness that forward motion cannot be maintained continuously and the ship comes to a stop after having crushed the ice under her forefoot. (Occasionally the ship may be partially beached on the unbroken ice with a portion of the forebody resting on the unbroken ice.) The ship, in such situations, is backed away from the ice an appropriate distance and then again moved forward against the ice. However, moving the breaker away from a beached position is often a difficult and tedious task which may include providing list and trim changes or assistance from other ships.
In current conventional icebreaker designs and maneuvers, substantially all broken ice remains in the water. Inasmuch as the largest pieces of broken ice that remain in the water may be of considerable size and weight, they continue to provide a hazard to the icebreaker hulls, propellers, and rudders. Smaller pieces of ice tend to move under the hulls to clog underwater hull openings such as sea chests and thruster ports.
The friction produced by movement of the ship's hull into the unbroken ice sheet and its snow cover, and through the broken ice that remains in the channel, as well as the additional energy required to overturn some chunks of ice, impedes the ship's progress and maneuverability.
Furthermore, the broken pieces of ice that are allowed to remain in the channel may refreeze and create additional work for the icebreaker to perform. If the ice pieces are allowed to refreeze in the channel, smaller icebreakers and other ships may continue to be vulnerable to the hazards produced by the ice-obstructed channel.
In addition to clearing a channel having a width equal to the width of the ship's beam, an icebreaker is often called upon for other duties such as channel widening, removing floating ice chunks, providing turnout points and turning basins, and harbor clearing. When required to perform these functions, contemporary icebreakers operate inefficiently due to the broken pieces of ice that remain in the water. The broken pieces of ice are even more hazardous to a ship that is backing and turning and otherwise maneuvering through the pieces of ice.
The conventional icebreaker, in use today, moves through the ice causing major radial cracks in the ice with the bow of the icebreaker. As the ship moves forward in the ice field, the shoulders of the bow and the forefoot, crush, turn, and submerge the ice in the water alongside the stem. Some ice may be deposited on the ice sheet, but much of the ice accumulates between the ship and the ice sheet and below the ship and below the ice sheet. Therefore, even though a channel has been made in the ice field, substantially all of the broken ice moves back into the channel behind the icebreaker and impedes the maneuverability of any other ship that may attempt to move through the channel formed by the icebreaker.